Abstract
Abstract. As the second largest area of contiguous tropical rainforest and second largest river basin in the world, the Congo Basin has a significant role to play in the global carbon (C) cycle. For the present day, it has been shown that a significant proportion of global terrestrial net primary productivity (NPP) is transferred laterally to the land–ocean aquatic continuum (LOAC) as dissolved CO2, dissolved organic carbon (DOC), and particulate organic carbon (POC). Whilst the importance of LOAC fluxes in the Congo Basin has been demonstrated for the present day, it is not known to what extent these fluxes have been perturbed historically, how they are likely to change under future climate change and land use scenarios, and in turn what impact these changes might have on the overall C cycle of the basin. Here we apply the ORCHILEAK model to the Congo Basin and estimate that 4 % of terrestrial NPP (NPP = 5800±166 Tg C yr−1) is currently exported from soils and vegetation to inland waters. Further, our results suggest that aquatic C fluxes may have undergone considerable perturbation since 1861 to the present day, with aquatic CO2 evasion and C export to the coast increasing by 26 % (186±41 to 235±54 Tg C yr−1) and 25 % (12±3 to 15±4 Tg C yr−1), respectively, largely because of rising atmospheric CO2 concentrations. Moreover, under climate scenario RCP6.0 we predict that this perturbation could continue; over the full simulation period (1861–2099), we estimate that aquatic CO2 evasion and C export to the coast could increase by 79 % and 67 %, respectively. Finally, we show that the proportion of terrestrial NPP lost to the LOAC could increase from approximately 3 % to 5 % from 1861–2099 as a result of increasing atmospheric CO2 concentrations and climate change. However, our future projections of the Congo Basin C fluxes in particular need to be interpreted with some caution due to model limitations. We discuss these limitations, including the wider challenges associated with applying the current generation of land surface models which ignore nutrient dynamics to make future projections of the tropical C cycle, along with potential next steps.
Highlights
As the world’s second largest area of contiguous tropical rainforest and second largest river, the Congo Basin has a significant role to play in the global carbon (C) cycle
Field data suggest that storage in tree biomass increased by 0.34 (0.15–0.43) Pg C yr−1 in intact African tropical forests between 1968 and 2007 (Lewis et al, 2009) due in large part to a combination of increasing atmospheric CO2 concentrations and climate change (Ciais et al, 2009; Pan et al, 2015), while satellite data indicate that terrestrial net primary productivity (NPP) has increased by an average of 10 g C m−2 yr−1 per year between 2001 and 2013 in tropical Africa (Yin et al, 2017)
Whilst the importance of land–ocean aquatic continuum (LOAC) fluxes in the Congo Basin has been demonstrated for the present day, it is not known to what extent these fluxes have been perturbed historically, how they are likely to change under future climate change and land use scenarios, and in turn what impact these changes might have on the overall C balance of the Congo
Summary
As the world’s second largest area of contiguous tropical rainforest and second largest river, the Congo Basin has a significant role to play in the global carbon (C) cycle. Field data suggest that storage in tree biomass increased by 0.34 (0.15–0.43) Pg C yr−1 in intact African tropical forests between 1968 and 2007 (Lewis et al, 2009) due in large part to a combination of increasing atmospheric CO2 concentrations and climate change (Ciais et al, 2009; Pan et al, 2015), while satellite data indicate that terrestrial net primary productivity (NPP) has increased by an average of 10 g C m−2 yr−1 per year between 2001 and 2013 in tropical Africa (Yin et al, 2017). A recent estimate of above-ground C stocks in tropical African forests, mainly in the Congo, indicates a minor net C loss from 2010 to 2017 (Fan et al, 2019). Recent field data suggest that the above-ground C sink in tropical Africa was relatively stable from 1985 to 2015 (Hubau et al, 2020)
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